The Impacts of Dams on Streamflow in Tributaries to the Lower Mekong Basin

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I have read the manuscript with great interest. Such multidisciplinary research can offer a comprehensive view of the dynamics that control tributary channels in the lower Mekong Basin associated with dam construction and anthropogenic pressures in this natural hydrographic system.

The methodology section outlines the various techniques, analyses, and data sources employed in the study in a clear way. The manuscript is written in a fairly comprehensive language and it is clear and informative. Furthermore, dams have become a major problem for rivers around the world, and this research contributes to knowledge on this matter.

however, some structural changes are necessary and I can suggest some of them:

- Figure 1 should be inserted in topic 2.1, where you talk about the study area. It is not usual to include a figure about the study area in the introduction, as there is a specific section.

- In my opinion, figure 2 can be considered as a result, and can fit satisfactorily into section 3.2 for example.

This suggestions does not change the fact that the figures are of good quality and illustrate the data well.

Regarding the literature used, they cover the theme and discussion of the work, but I missed work carried out in this context in other regions of the world, which could be included in the introduction in order to improve the state of the art. The list is long, and I will suggest some of them that I consider important on the issue of dams and river dynamics in general:

https://doi.org/10.3390/jmse10111585

The conclusions must be improved, the authors must highlight the progress of this study concerning the theme, talk about future perspectives ones, and whether the methodology used was sufficient to achieve the proposed objectives.

After making the corrections indicated above, the manuscript will reach the expected quality to be accepted for publication in Sustentabilidade.

Comments on the Quality of English Language

The quality of English is adequate and as expected.

Author Response

Dear Editor and Reviewers,

Please find the attached our revised manuscript title “The Impact of Dams on Streamflow in Tributaries to the Lower Mekong Basin”, Ref: sustainability-3021569, that we would like to resubmit to Sustainability. We have revised our article following reviewer’s comments and suggestions, which are valuable and insightful. We hope the revised manuscript meet your expectations and can be accepted for publication in Sustainability.

Please find below the point-by-point responses.

Sincerely,

Romduol Khoeun and Co-authors

Point-by-point responses:

Reviewer 1:

• Comment 1: Figure 1 should be inserted in topic 2.1, where you talk about the study area. It is not usual to include a figure about the study area in the introduction, as there is a specific section.

The authors moved the figure 1 into topic 2.1 of the revised manuscript.  

• Comment 2: In my opinion, figure 2 can be considered as a result, and can fit satisfactorily into section 3.2 for example

We thank for this suggestion. However, after careful discussion among all authors, we agreed to still keep the figure and accompanying text in this method section because it is not the result of our study. We presented here to visualize the rational of our study periods selection, the pre-impact and the post-impact, during which the dams boomed.

• Comment 3: Regarding the literature used, they cover the theme and discussion of the work, but I missed work carried out in this context in others regions of the world, which could be included in the introduction in order to improve the state of the art. The list is long, and I will suggest some of them that I consider important on the issues of dams and river dynamics in general: https://doi.org/10.3390/jmse10111585

We have now included the literature review from other river basins to improve the Introduction part. Please see more in lines 68-74.

• Comment 4: The conclusion must be improved, the authors must highlight the progress of the study concerning the theme, talk about future perspectives ones, and whether the methodology used was sufficient to achieve the proposed objective.

Thank you for this insightful suggestion to improve the conclusion. The authors incorporated the progress of the study theme, and future perspectives to improve the conclusion. Please see lines 290-306 of the revised manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

See attached document. 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Very minor issues. 

Author Response

Dear Editor and Reviewers,

Please find the attached our revised manuscript title “The Impact of Dams on Streamflow in Tributaries to the Lower Mekong Basin”, Ref: sustainability-3021569, that we would like to resubmit to Sustainability. We have revised our article following reviewer’s comments and suggestions, which are valuable and insightful. We hope the revised manuscript meet your expectations and can be accepted for publication in Sustainability.

Please find below the point-by-point responses.

Sincerely,

Romduol Khoeun and Co-authors

Point-by-point responses:

Reviewer 2:

• Comment 1: L38; consider rephrasing, these are not regions but rivers?

The authors modified in to rivers.

• Comment 2: L42; typo, ‘destroy wetland destruction?

We modified in to “contribute wetland destruction”.

• Comment 3: L48; Is this meant to qualify the preceding sentence?

We didn’t intend to qualify the preceding sentence, rather we intended to elaborate more about the impact of dams. To clarify the meaning of the sentence, we modified it to “… and therefore flow magnitude, timing, frequency, duration, and rate of change can be also affected [19, 20], which consequently leads to longer wet or dry seasons.”

• Comment 4: L69; What is “whole 3S”?

It meant “whole 3S Basin”, comprising the Sekong, Sesan and Srepok Rivers. We now modified it to “whole 3S Basin”.

• Comment 5: L65; Figure 1; It might be useful to highlight the rivers in question so that readers are able to easily identify them on the map.

We have modified accordingly.

• Comment 6: L95; How is it instructive to compare the hydrologic alteration for the two rivers?

We thank the reviewer for this insightful comment. Our intention is to compute the extent hydrologic alteration differs between the two rivers, given that this situation happen in every dammed river. To clarify the meaning of the sentence, we modified it to “We then compare hydrologic alteration between the two periods for each river and between the two rivers to compute the extent hydrologic alteration differs”. This analysis will can provide a broader context to understand the impact of dam operations on downstream flow regimes.

• Comment 7: L130-133: The authors need to rationalize why they chose only 10 of the 33 parameters of the 33 normally used in the IHA-why are the 10 parameters adequate for the analysis and able to explain observed changes? This has not been explained, unless I misread it.

We have provided a rational this selection as “We selected 10 parameters as we focused on the magnitude and duration in different periods, while the Change in Extreme Flow Events was detected by the flow duration curves and flow maxima and minima”. Please see in line 146-148.

• Comment 8: L149; Check phrasing, “were then were divided into….”

We have deleted the first “were” from the sentence.

• Comment 9: L184-185; Can the authors need to rationalize this or provide some sort of explanations as to the negligible change for the 90-day maximum? Such statements need qualification. Again, Lines 186-188

We have now elaborated as … “because it was only 1% higher in the post-impact compared to the pre-impact periods.” and … “because a negative relative change of -10% was observed” in the revised manuscript, lines 197-198 and line 202-204.

• Comment 10: L190; Would it not be useful to have both the pre and post impact curves on the graph-this would make it easier for readers to see the comparison?

We thank the reviewer for this great idea. In figure 4, the relative changes were computed for the minima and maxima metrics (10 variables on the x-axis). For the pre- and post-impact curves (we understand this as the flow) combined together, it will hard and complicate to do comparison, in particular when comparing the relative change between the two rivers. Nevertheless, we provide the detailed figure as an appendix Figure A2 in the revised manuscript.

• Comment 11: L 196-208; All this needs some sort contextualization/rationalization. It should be assumed that the changes in stream flow are not a function of natural processes, likely due to artificial control. So, what exactly is the study trying to discover/elucidate? The authors have touched on this in the discussion section-which needs to be more convincing, it is not, as it stands.

We thank the reviewer for this deep thought comment. In our paper, we aimed to quantify and compare hydrologic change between the pre- and post-periods of the least and most dammed rivers, with the expectation that there is a significant hydrologic change between the two periods and that the change is more profound in the most dammed river. This is the first study ever conducted in the system. As a result, we found that hydrologic change increased significantly in the post-impact period for the two rivers in the dry season, but higher degree of change was found in Sekong River (the least dammed) than in the Sesan (the most dammed). Whereas there was no significant change found for the two rivers in wet season.

We hope that this explanation, which is already adapted in the revised manuscript lines: 212-228, meets your expectation.

Comment 12: See above (lines 196-208), this should be assumed, and if no explicit insights are provided, then this whole paper is reduced to simple reporting of observations-what is novel or what does it add to what is already there. The authors may want to be explicit about the objectives of the study, why is worthy of publishing.

Thank you again the reviewer for this comment. And as we have explained earlier, our study is the first of its kind to compare the hydrologic change between the pre- and post-periods of the least and most dammed rivers and then compare the change between the two rivers. Our result is interesting as we found the significant change occurred in the dry season, but not in the wet season either in the least or most dammed river, as detailed in lines: 212-228.

• Comment 13: L243; Where is this coming from? It is not given, and the reader should not be left to speculate as to why the stated factors explain the observations. It is incumbent on the authors to tell a convincing story. It must be empirically grounded, and if there are supporting studies, then the authors ought to cite the same?

Thank the reviewer for reminding us to follow the scientific integrity and providing credits to previous finding. We have cited this synthesized sentence in the revised manuscript.

• Comment 14: L251; The said impacts have not been made explicit.

In the revised manuscript, we have made this paragraph more explicit by providing key examples that have been found from previous studies. Please see lines: 300-301 in the revised manuscript.

Reviewer 3 Report

Comments and Suggestions for Authors

The implemented methodology is clear, well explained and implemented.

However, my comments are mainly focused in the results section.

Lines 48-49: “Extended wet or dry periods may occur [19, 20]”. This statement is unclear. What is a dry or a wet period of a dam?

Lines 41-42. Apart from single-purpose dams, there are also multipurpose dams that serve various water uses. Therefore, I recommend adding the following reference to enhance the manuscript: Skoulikaris, C., & Ganoulis, J. (2017). Multipurpose hydropower projects economic assessment under climate change conditions. Fresenious Environ. Bull, 26, 5599-5607.

Line 116: The authors use a 4-year dataset for their analysis, specifically PreIm (1965-1968) and PostIm (2018-2021). How did the authors ensure that the data from these periods are comparable, particularly in terms of similar atmospheric characteristics (i.e. both datasets correspond to similar wet periods) ? Are these 4-year periods representative of the entire basin?

Lines 169-170: “There was a larger reduction in streamflow between the pre- and post-impact periods for Q5 (the highest 5% of flows) than for Q95 (the lowest 5% of flows), and for the Sekong River than the Sesan River (Figure 3)”. Was this outcome not expected, as the smoothing of peak flows is characteristic of dammed rivers?

Lines 177-188: In Figure 4, we can see that the minimum flow increase is quite significant, while the maximum flows, as the authors mention, were 50% higher than in the pre-dam impact period. However, this contradicts the statement that peak flows are currently smaller than in the pre-dam period (Figure 3 and relevant text).

In the discussion section, I recommend mentioning that small run-of-river hydropower dams are a safe and relatively environmentally friendly approach, as clearly presented in the following research: "Run-of-river small hydropower plants as hydro-resilience assets against climate change." This is a good opportunity to add a paragraph about the observed negative (or positive) impacts of the current hydrosystem status, and further discuss on it.

In the discussion section, the authors should mention the added value of this manuscript compared to the numerous existing studies in the specific case area. What are the unique contributions of this research? Additionally, the authors should discuss the limitations of their study and suggest potential directions for further enhancement and future research.

The conclusion part should be enhanced.

Author Response

Dear Editor and Reviewers,

Please find the attached our revised manuscript title “The Impact of Dams on Streamflow in Tributaries to the Lower Mekong Basin”, Ref: sustainability-3021569, that we would like to resubmit to Sustainability. We have revised our article following reviewer’s comments and suggestions, which are valuable and insightful. We hope the revised manuscript meet your expectations and can be accepted for publication in Sustainability.

Please find below the point-by-point responses.

Sincerely,

Romduol Khoeun and Co-authors

Point-by-point responses:

Reviewer 3:

• Comment 1: L48-49; “Extended wet or dry periods may occur [19, 20]. This statement is unclear. What is a dry or a wet period of a dam?

This sentence is now deleted, and we adapted the previous sentence to “… which consequently leads to longer wet or dry seasons.”, which we wanted to refer to the “extended periods” in the submitted version of the manuscript.

• Comments 2: L41-42; Apart from single-purpose dams, there are also multipurpose dams that serve various water uses. Therefore, I recommend adding the following reference to enhance the manuscript: Skoulikaris, C.,& Ganoulis, J. (2017). Multipurpose hydropower projects economic assessment under climate change conditions. Fresenious Environ. Bull, 26, 5599-5607

We appreciate this insightful comment. To address this, we added the following the sentence to the manuscript: “In addition to single purpose dams, there are also multipurpose dams that serve various water used such as flood control, irrigation, and electricity generation (Skoulikaris & Ganoulis, 2017).” Please see in lines 42-43.

• Comment 3: L116; The authors use a 4-year dataset for their analysis, specifically PreIm (1965-1968) and PostIm (2018-2021). How did the authors ensure that the data from these periods are comparable, particularly in terms of similar atmospheric characteristics (i.e. both datasets correspond to similar wet periods)? Are these 4-years periods representative of the entire basin?

Thank the reviewer this comment. We understand this concern. However, we would like to clarify that our analysis was largely based on the two hydrological data sets, with the first collected from a more natural state (as indicated in Figure 2) and the second collected from a more impacted state, in particular the hydrology of the two rivers.

The two rivers shared common watershed borders and located in a tropical region with similar atmospheric conditions. Due to geographic and climatic influences, the atmospheric characteristics of these rivers are largely comparable. The hydrological gauges recorded at the outlets of these two rivers captured and represented the entire basin. This ensures that, even in the absence of atmospheric data, our finding is insightful into the hydrologic change and is reflective of a broader change after the boom of hydropower dam development in the two rivers.

We hope this clarifies our approach and provides reassurance regarding the suitability of the data selected for our analysis.

• Comment 4: L169-170; “There was a larger reduction in streamflow between the pre- and post-impact periods for Q5 (the highest 5% of flows) than for Q95 (the lowest 5% of flows), and for the Sekong River than the Sesan River (Figure 3)”. Was this outcome not expected, as the smoothing of peak flows is characteristic of dammed rivers?

This is a good technical question, and linked to the next comment. We made a mistake on the important finding of our study. We intended to indicate a “larger increase”, not “larger reduction”. We have now revised this part accordingly. Please see lines 181-183.

• Comment 5: L177-188; In figure 4, we can see that the minimum flow increase is quite significant, while the maximum flows, as the authors mention, were 50% higher than in the pre-dam impact period. However, this contradicts the statement that peak flows are currently smaller than in the pre-dam period (Figure 3 and relevant text).

This is a very good technical but important comment, and thank the reviewer for the comments. We did make a mistake on the meaning of the statement. We meant there was a “larger increase”, not “larger reduction”. We have now revised the meaning accordingly, please see lines: 181-183.

• Comment 6: In the discussion section, I recommend mentoring that small run-of-river hydropower dams are a safe and relatively environmentally friendly approach, as clearly presented in the following research “Run-of-river small hydropower plants as hydro-resilience assets against climate change. “This is a good opportunity to add a paragraph about the observed negative (or positive) impacts of the current hydro system status, and further discuss on it

We appreciate this constructive comment. We adapted this accordingly, please see lines 268-275 in the revised manuscript. 

• Comment 7: In the discussion section, the authors should mention the added value of this manuscript compared to the numerous existing studies in the specific case area. What are the unique contributions of this research? Additionally, the authors should discuss the limitations of their study and suggest potential directions for further enhancement and further research.

We indicated that “Our study is the first to assess and compare the impacts of dams on streamflow between the least and most dammed rivers in the 3S Basin”, because previous study focused on just one single river or the whole 3S basin. Moreover, we have included the limitation and future work direction as suggested, please see lines: 276-28.

• Comment 8: The conclusion part should be enhanced.
  We have now improved the conclusion part as can be found in the revised manuscript. We hope this revised part of the conclusion meets your expectation.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

None

Comments on the Quality of English Language

The manuscript needs considerable editing - odd sentences and typos in numerous places. Ensure that statements convey the intended meaning. 

Reviewer 3 Report

Comments and Suggestions for Authors

The authors enhanched their manuscript according to the reviewers comments.